Probing the superconducting gap symmetry of ${\mathrm{PrRu}}_4{\mathrm{Sb}}_{12}:$ A comparison with ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of ${\mathrm{PrRu}}_4{\mathrm{Sb}}_{12}$ down to 0.1 K. Both $\lambda$ and superfluid density ${\rho }_s$ exhibit an exponential behavior for $T<\mathrm{}{0.5T}_c,$ with parameters $\Delta {\mathrm{}\left(0\right)/k}_B{T}_c=1.9\mathrm{}$ and $\lambda \mathrm{}\left(0\right)=2900\mathrm{}\mathrm{\AA }.$ The value of $\Delta \mathrm{}\left(0\right)\mathrm{}$ is consistent with the specific-heat jump value of $\Delta C/\gamma T_c=1.87\mathrm{}$ measured elsewhere, while the value of $\lambda \mathrm{}\left(0\right)\mathrm{}$ is consistent with the measured value of the electronic heat-capacity coefficient $\gamma .$ Our data are consistent with ${\mathrm{PrRu}}_4{\mathrm{Sb}}_{12}$ being a moderate-coupling, fully gapped superconductor. We suggest experiments to study how the nature of the superconducting state evolves with increasing Ru substitution for Os.